High-speed hydraulically operated presses for plastics and metal processing are known, in which these machines are designed for varying tool heights and opening strokes. For example, the working cylinder stroke can be large enough that, depending on the tool use in the clearance between the press table and the press platen, the maximum tool height (a large gap between the mounting plates of the press table and the press platen) to be expected can be used both with a stroke necessary for the maximum tool height and a very small tool height (small gap between the mounting plates of the press table and the press platen). Due to a necessary oil column of up to 2.5 meters, the corresponding working surface, and the high specific processing pressure in the tools, such types of presses have a very large oil volume in the working cylinders. Aside from a large compression volume, such types of presses also result in economic disadvantages during their operation.
There are now various approaches for limiting the no-load stroke of the working cylinder for different tool applications and nevertheless ensuring a high flexibility of the press with short change-over times. For example presses are known according to DE 36 37 544 A1 and DE 36 37 545 A1, in which, with frictionally engaged clamps on the steel columns (round or square shape), the working stroke is supposed to be reduced to a so-called short stroke with the least compression volume. However, these frictionally engaged connections have not proven worthwhile in practice or are too expensive.
Of course, there also are a plurality of other mechanical solutions for a height adjustment of long stroke presses in prior art. Systems where the force is transmitted likewise with short-stroke working cylinders, such as in injection molding machines with retractable crossbars, have not proven worthwhile because of the large pressing plates to be moved, the great distances, and a prolongation of the cycle time resulting therefrom.
A possible remedy lies in DE 10 2004 040 512 A1, in which a rotary table with a driving shaft and four legs is in each case provided between a short stroke piston and the legs of the press platen. The legs are adjustably arranged by means of a rotary drive of the driving shaft and a torque angle, either in a power transmission position onto the hook suspension faces of the web plates or in an opening position between the web plates of the press platen.
In U.S. Pat. No. 4,304,540 A, a press executed with columns is known, in which the press platen moves on several externally arranged columns. The working cylinders are arranged in the press table and clamping jaws are accommodated above the press platen that connect the press platen to the columns in an interlocking manner after bridging the no-load stroke and consequently close the flux for the pressing procedure. The control of the flux has proven disadvantageous for the most part because the flux diversion takes place in the grating of the columns so that the peaks of the load have to be diverted in notched machine elements. The gratings of the columns are also freely accessible and consequently exposed to possible contamination. The clamping of the columns takes place at stoppage.
Aside from the mechanical alternatives, an attempt was made to shorten the no-load stroke via other drives, such as for instance, electrically operated intermediate drives.
The disadvantage arising here is that spikes of several hundred amperes occur in the driving motors, because the drives used must move heavy loads rapidly and usually without intermediate gear ratios (as in crane loads). It is such types of spikes that are measured by electricity suppliers and charged high surcharges when calculating the electric bill. The surcharges for massive peak loads such as these are so high that they significantly affect the economics calculation for a press. In addition, the appropriate power supplier may impose upon the operator to install devices for damping the peak loads for the supply network, which in turn incur high acquisition and maintenance costs.
Thus, the following disadvantages arise in the known prior art:
a) large hydraulic oil quantities must be moved in long stroke presses, which result in increased expenditure of energy and increased wear and tear of the hydraulic elements (oil consumption, pumps, valves),
b) in mechanical no-load stroke bridging, to avoid the disadvantages in a), there are awkward clamps that impair the closing speed of the press and prolong the working cycle,
c) mechanical no-load stroke bridging is normally subject to increased wear due to unfavorable fluxes in the mechanical clamp,
d) the columns or connecting devices for the engagement of the mechanical clamping elements are normally unprotected and open to normal contamination in the workshop halls, and
e) no-load stroke bridging by means of electric drives result in high spikes that are disadvantageous to the profitability of the operation and increased control engineering requirements with regard to the energy supply.